This invention relates to a method of increasing the strength of a paper mat of fibers produced in a papermaking process. Paper mat comprises water and solids and is commonly 4 to 8% water. The solid portion of the paper mat includes fibers (typically cellulose based fibers) and can also include filler. Increasing the strength of the paper mat would allow one to increase the proportion of the solids that is filler content. This is desirable because it reduces raw materials costs, reduces energy needed in the papermaking process, and increases the optical properties of the paper. Prior Art discloses paper mat having a solid portion of between 10% and 40% filler. The Prior Art however also discloses that increasing the filler content coincides with a loss in strength in the resulting paper.
Fillers are mineral particles that are added to paper mat during the papermaking process to enhance the resulting paper's opacity and light reflecting properties. Some examples of fillers are described in U.S. Pat. No. 7,211,608. Fillers include inorganic and organic particle or pigments used to increase the opacity or brightness, reduce the porosity, or reduce the cost of the paper or paperboard sheet. Some examples of fillers include one or more of: kaolin clay, talc, titanium dioxide, alumina trihydrate, barium sulfate, magnesium hydroxide, pigments such as calcium carbonate, and the like. Previous attempts to increase the filler content in paper without losing paper strength are described in British Patent GB 2016498, and U.S. Pat. Nos. 4,710,270, 4,181,567, 2,037,525, 7,211,608, and 6,190,663.
Calcium carbonate filler comes in two forms, GCC (ground calcium carbonate) and PCC (precipitated calcium carbonate). GCC is naturally occurring calcium carbonate rock and PCC is synthetically produced calcium carbonate. Because it has a greater specific surface area, PCC has greater light scattering abilities and provides better optical properties to the resulting paper. For the same reason however, PCC filled paper mat produces paper which is weaker than GCC filled paper.
Paper strength is a function of the number and the strength of the bonds formed between interweaved fibers of the paper mat. Filler particles with greater surface area are more likely to become engaged to those fibers and interfere with the number and strength of those bonds. Because of its greater surface area, PCC filler interferes with those bonds more than GCC.
As a result, papermakers are forced to make an undesirable tradeoff. They must either choose to select a paper with superior strength but inferior optical properties or they must select a paper with superior optical properties but inferior strength. Thus there is a clear need for a method of papermaking that facilitates a greater amount of filler in the paper, a paper that has a high opacity, and a filled paper that has a high degree of strength.